Refactor wp.media.gallery.attachments to accept an array of attachment ids rather than shortcode

Description

In /wp-includes/js/media-editor.js, line 175, the wp.media.gallery.attachments method is too closely coupled to the shortcode. This makes it impossible to fetch Attachments without first creating a shortcode to pass in.

A better approach would be to accept an array of attachment ids or a hash of properties; more than just shortcodes need to fetch Attachments by their ids.